docs: generate deltachat-rpc-client documentation

To preview the docs, run:
```
scripts/build-python-docs.sh
firefox dist/html/index.html
```

I have removed the Makefile because modern Sphinx Makefile is just a
wrapper for `sphinx-build -M`:
3596590317/sphinx/templates/quickstart/Makefile.new_t
and sphinx-quickstart even has an option `--no-makefile`.
`make.bat` makes even less sense.
In `scripts/build-python-docs.sh` I use `sphinx-build` directly
without `make` wrapper.

python/doc/cffi/plugins.rst

@@ -0,0 +1,38 @@
+
+Implementing Plugin Hooks
+==========================
+
+The Delta Chat Python bindings use `pluggy <https://pluggy.readthedocs.io>`_
+for managing global and per-account plugin registration, and performing
+hook calls. There are two kinds of plugins:
+
+- Global plugins that are active for all accounts; they can implement
+  hooks at account-creation and account-shutdown time.
+
+- Account plugins that are only active during the lifetime of a
+  single Account instance.
+
+
+Registering a plugin
+--------------------
+
+.. autofunction:: deltachat.register_global_plugin
+    :noindex:
+
+.. automethod:: deltachat.account.Account.add_account_plugin
+    :noindex:
+
+
+Per-Account Hook specifications
+-------------------------------
+
+.. autoclass:: deltachat.hookspec.PerAccount
+    :members:
+
+
+Global Hook specifications
+--------------------------
+
+.. autoclass:: deltachat.hookspec.Global
+    :members:
+